The present invention relates generally to touchscreens and, more particularly, to a method and apparatus for adapting the frequency of a touchscreen controller in order to match the controller to the particular operating characteristics of a specific touchscreen.
Touchscreens are used in conjunction with a variety of display types, including cathode ray tubes (i.e., CRTs) and liquid crystal display screens (i.e., LCD screens), as a means of inputting information into a data processing system. When placed over a display or integrated into a display, the touchscreen allows a user to select a displayed icon or element by touching the screen in a location corresponding to the desired icon or element. Touchscreens have become common place in a variety of different applications including, for example, point-of-sale systems, information kiosks, automated teller machines (i.e., ATMs), data entry systems, etc.
In one specific type of touchscreen, an acoustic touchscreen, acoustic or ultrasonic waves are generated and directionally propagated across the touchscreen surface utilizing the phenomena of surface acoustic waves, e.g., Rayleigh waves, Love waves, or other waves. Typically each axis of the touch panel includes a single transmitter transducer, a single receiver transducer, and a pair of reflective arrays. The transmitting transducers and the receiving transducers are coupled to a controller, the controller generating the drive signals that are applied to the transmitting transducers and amplifying, conditioning and responding to the signals from the receiving transducers. The acoustic wave produced by each transmitter transducer is reflected by the reflective array located near the touchscreen edge. The array reflects the acoustic wave, typically at a right angle along the entire length of the array, producing a surface acoustic wave pattern that propagates across the active area of the touchscreen. The propagated surface acoustic wave has a substantially linear wavefront with a uniform amplitude. The opposing reflective array reflects the surface propagated acoustic wave to a receiving transducer. By monitoring the arrival time and the amplitude of the propagated wave along each axis of the touchscreen, the location of any wave attenuation point on the touchscreen surface can be determined. Attenuation can be caused by touching the screen with a finger or stylus or other media.
Typically a manufacturer of touchscreen systems produces or purchases controllers with a predetermined oscillation frequency that is within a well defined frequency range, the reference frequency being provided by a crystal oscillator. Then during the manufacturing process the characteristic frequency of each touchscreen is determined and adjusted, as necessary, to ensure that there is sufficient match between the touchscreen and the oscillation frequency of the controller.
Let us more carefully define the characteristic frequency of a touchscreen. Acoustic touchscreens of the types of interest here have the property of being a narrow band pass filter. The center frequency of the narrow band is determined by the spacing of the reflectors and by the velocity of the acoustic waves. As a consequence, a brief burst applied to a transmitter transducer appears, after a time delay corresponding to an acoustic wave traveling the shortest possible path to a receiving transducer, in the form of a long drawn-out wave train. While the frequency spectrum of the input burst is typically quite wide due to the short duration of the burst, the spectrum of the output wave train is ideally very narrow and sharply peaked at a specific frequency. This specific frequency is referred to as the touchscreen""s characteristic frequency. It is desired that the touch system""s operating frequency match the touchscreen""s characteristic frequency.
In principle, an ideal touchscreen has a single characteristic frequency. In practice, manufacturing variations can result in a plurality or range of characteristic frequencies. Current practice involves making a sufficient investment in the touchscreen manufacturing process so that there is effectively only a single characteristic frequency of the touchscreen and that this characteristic frequency matches that determined by the controller""s reference oscillator. In order to achieve the desired control over the touchscreen manufacturing process, precise coordination of array design, careful monitoring of the supply chains of incoming materials, and prompt electronic testing of reflective arrays are required. In addition, when an unanticipated change or variation is discovered, rapid corrective action is necessary. For example, the array may need to be redesigned and a new printing mask fabricated. The degree of coordination, monitoring, and testing required to maintain control of the touchscreen characteristic frequency adds cost to the process and limits production to facilities with a workforce well trained in the intricacies of acoustic touchscreen manufacture. This is an important limitation of present acoustic touchscreen technology.
In general, frequency mismatch can be categorized as being either global or localized in nature. In cases in which the frequency mismatch is global, the source of mismatch affects the entire touchscreen. For example, if the reference oscillator of a controller drifts, or alternatively, if the glass substrate has an unexpected acoustic velocity (e.g., due to the glass substrate being fabricated by a different glass supplier), the frequency match between the touchscreen and the controller is compromised regardless of the location of interest on the touchscreen. In contrast, in cases in which the frequency mismatch is localized, only a specific region of the touchscreen may exhibit mismatch with the controller.
Both global and localized frequency mismatch can be caused by a variety of sources. Although some sources of mismatch can be overcome through sufficient quality control, often the cost of such control can be quite high. For example, variations in the touchscreen glass substrate can vary the acoustic wave velocity thereby causing global frequency mismatch, controlling the glass supply chain and manufacturing process sufficiently to ensure that the acoustic wave velocity of all substrates fall within a narrow range may be economically unfeasible. Controlling the glass supply chain and manufacturing process is even more problematic in those instances in which acoustic reflective arrays are printed directly onto the faceplate of a cathode ray tube (i.e., CRT). Specific glass characteristics that are difficult to control to the degree necessary to avoid global frequency mismatch include the chemical composition and the thermal history (e.g., annealing time and temperature, etc.).
Another source of frequency mismatch is due to undesired variations within the reflective array printed on the touchscreen substrate. These variations may, for example, result from the array mask being distorted during the screen printing process. Print mask distortion is especially problematic if the array is to be printed directly onto a CRT faceplate. Other array printing techniques such as pad printing are also subject to the registration errors introduced during the printing process that can lead to further frequency mismatch. Another source of frequency mismatch can arise from improperly correcting for the spherical geometric effects of a non-planar substrate surface.
What is needed in the art is a method and apparatus for adapting the oscillation frequency of a controller to the operating frequency requirements of specific touchscreens. The present invention provides such a method and apparatus.
The present invention provides a method and apparatus for adapting the frequency of a controller to the operating frequency requirements of a specific touchscreen substrate, wherein the touchscreen substrate includes reflective arrays. More specifically, the controller is adapted such that it outputs a burst signal to the touchscreen""s transmitting transducers or conditions the signal from the touchscreen""s receiving transducers, thereby accommodating the particular operating frequency characteristics of the touchscreen""s substrate.
In one application of the invention, the characteristic frequency or frequencies of a specific touchscreen is first determined. The frequency of the controller that is intended for use with this substrate is then adjusted to match the substrate""s measured characteristic frequency or frequencies thus allowing the two components to be paired as a matched set. In an alternate application, a touchscreen substrate is paired with a controller prior to matching the operating frequencies of the two components. After pairing, the system is initialized during which time the touchscreen substrate""s frequency characteristics are determined. The frequency characteristics of the controller are then adjusted to match those of the substrate. If desired, the system can periodically retest the frequency characteristics of the substrate and readjust the controller""s output as deemed necessary.
In one embodiment of the invention that is primarily intended to compensate for global frequency mismatch errors, the adaptive controller of the invention uses analog signal processing and a crystal reference oscillator. A digital multiplier is used to modify the output of the reference oscillator to generate the desired frequency of the tone burst sent to the transmitting transducers and/or to vary the frequency used by the receive circuit to produce the base-band signal. The burst length is determined by a burst circuit. The desired operating frequency is determined by a mixer containing circuit that compares the output of the digital multiplier to the suitably conditioned output signal of the receiving transducer. The output from the mixer containing circuit is used to determine the desired operating frequency.
In another embodiment of the invention that is intended to compensate for both global and local frequency variations, the adaptive controller of the invention uses digital signal processing and a crystal reference oscillator. In this embodiment a digital signal processor receives the digitized, filtered outputs from a pair of mixers. The inputs to the mixers are a pair of reference signals, one of which has been phase shifted by 90 degrees, and suitably filtered and amplified receiver transducer RF signals. This embodiment is an example of the use of a phase-sensitive controller in which the complete mathematical content, e.g., phase and amplitude, of the received signal is digitized. With complete digitized information available for processing by digital signal processor algorithms, software tunable frequency filters can be applied to the received signal. The digital signal processor, based on correction values contained in memory, applies a frequency filter with a specific center frequency which preferably varies according to the delay time since the last burst was transmitted. Thus the system can adapt to variations caused by localized variations in the acoustic wave reflective array.
In yet another embodiment of the invention, a non-crystal local oscillator is used to provide the reference signal in the adaptive controller. The use of such an oscillator allows the controller to be miniaturized to a sufficient extent to allow it to be mounted directly to a touchscreen substrate. A feedback loop is used to compensate for the drift of the oscillator. In this embodiment the conditioned RF signal from the touchscreen receiver transducers is mixed with the output from the local oscillator. The IF output from the mixer is passed to a discriminator circuit that generates a voltage, the sign of which depends on whether the frequency is higher or lower than desired and the amplitude of which depends on the degree of deviation from the desired frequency. The output from the discriminator is used to adjust the frequency of the local oscillator such that it tracks the frequency of the touchscreen. To achieve the desired burst frequency, the stabilized output from the local oscillator is mixed with the output from an IF oscillator.
In yet another embodiment of the invention, the burst is sufficiently broadband so that it is sufficient to adjust only the center frequency of the circuitry processing the receive circuit by means of a voltage controlled, variable frequency bandpass filter.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.